ABUNDANCE ANOMALY OF THE 13C ISOTOPIC SPECIES OF c-C3H2 IN THE LOW-MASS STAR FORMATION REGION L1527

The rotational spectral lines of c-C3H2 and two kinds of the C-13 isotopic species, c-(CCCH2)-C-13 (C-2v symmetry) and c-(CCCH2)-C-13 (C-s symmetry), have been observed in the 1-3 mm band toward the low-mass star-forming region L1527. We have detected 7, 3, and 6 lines of c-C3H2, c-(CCCH2)-C-13, and c-(CCCH2)-C-13, respectively, with the Nobeyama 45 m telescope. and 34, 6, and 13 lines, respectively, with the IRAM 30 m telescope, where seven, two, and two transitions, respectively, are observed with both telescopes. With these data, we have evaluated the column densities of the normal and C-13 isotopic species. The [c-C3H2]/[c-(CCCH2)-C-13] ratio is determined to be 310 +/- 80, while the [c-C3H2]/[c-(CCCH2)-C-13] ratio is determined to be 61 +/- 11. The [c-C3H2]/[c-(CCCH2)-C-13] and [c-C3H2]/[c-(CCCH2)-C-13] ratios expected from the elemental C-12/C-13 ratio are 60-70 and 30-35, respectively, where the latter takes into account the statistical factor of 2 for the two equivalent carbon atoms in c-C3H2. Hence, this observation further confirms the dilution of the C-13 species in carbon-chain molecules and their related molecules, which are thought to originate from the dilution of C-13(+) in the gas-phase C+ due to the isotope exchange reaction: C-13(+) + CO -> (CO)-C-13 + C+. Moreover, the abundances of the two C-13 isotopic species are different from each other. The ratio of c-(CCCH2)-C-13 species relative to c-(CCCH2)-C-13 is determined to be 0.20 +/- 0.05. If C-13 were randomly substituted for the three carbon atoms, the [c-(CCCH2)-C-13]/[c-(CCCH2)-C-13] ratio would be 0.5. Hence, the observed ratio indicates that c-(CCCH2)-C-13 exists more favorably. Possible origins of the different abundances are discussed.